Fiber tow laying machines and fiber tape laying machines are well known in the art and enjoy increasing usage to produce composite plastic parts, especially in aerospace applications, to replace comparable metallic parts. These composite plastic parts advantageously have a high strength to weight ratio, are producible in complex shapes that eliminate the need for several individual metallic parts, exhibit corrosion resistance and have other desirable physical properties (e.g., low electrical and heat conductivity). Various fiber tow laying machines and improvements thereto have been described in the literature of the art and the distinctions and advantages of these machines over filament winding and tape laying machines has been well documented (see for example U.S. Pat. No. 4,699,683 to McCowin and U.S. Pat. No. 5,022,952 to Vaniglia). Fiber tow laying machines can individually feed and cut separate fiber bundles or tows forming a fiber band being laid down on a work surface or form. This selective cutting and feeding of tows advantageously allows the fiber placement head to put down the tows in an arcuate path on the work surface that prevents buckling, wrinkling or misalignment of fibers. The fiber tows, also known as tow pregs, are generally a bundle of continuous fibers impregnated with a resin (i.e., a polymeric material that may be in a cured, uncured or partially cured state).
Many improvements have been made to fiber tow laying machines and fiber tape laying machines, and these improvements have been described in the literature of the composite plastics art, especially the patent literature. One of these improvements has been the segmented presser member assembly. The segmented presser member assembly generally has a series of side-by-side parallel arranged individual linearly movable presser elements that engage fiber tows to lay and compact the tows onto a work surface or form. The presser elements, which may be nonrotating or rotating elements, usually travel above and below a datum line. This improvement permits the presser member of the fiber tow laying and compacting head of a fiber tow laying machine to more easily and readily conform to changes in the contour of the work surface or form during the lay up procedure. Such segmented presser members, for fiber tow laying machines, have been described, for example, in U.S. Pat. No. 4,292,108 to A. E. Weiss et.al., U.S. Pat. No. 4,601,775 to R. J. Grone, U.S. Pat. No. 4,867,834 to B. A. Alenskis et.al., U.S. Pat. No. 4,869,774 to J. D. Wisbey, U.S. Pat. No. 5,015,326 to E. Frank, U.S. Pat. No. 5,045,147 to V. M. Benson et.al. and U.S. Pat. No. 5,110,395 to M. M. Vaniglia. The prior art segmented presser members commonly employ an elastic sleeve to cover and stretch across the fiber tow engaging faces of all the presser elements of the segmented presser member to form a continuous elastic or resilient fiber tow engaging surface for laying and compacting the fiber tows onto a work surface. This elastic sleeve is usually made of an elastomeric material such as elastomeric polyurethane. As the contour of the work surface changes during the lay up procedure the elastic sleeve would deform in response to the changing contour. This deformation of the sleeve is mirrored by linear movement of individual presser elements contacting the sleeve so that the presser member maintains contact with the work surface during changes in the surface contour. The segmented presser member and elastic sleeve combination improved the lay down and compaction of fiber tows onto a work surface because of the improved conformation of the presser member to changes in the work surface contour and improved contact of the presser member with the work surface.
Although improvements in the laydown and compaction of fiber tows onto a work surface are achieved with the combination of the segmented presser member and elastic sleeve, this combination produces other problems. One of the problems is the added spring force that the common sleeve induces on the presser elements as they are forced to move against the sleeve especially during compaction of the tows. This force varies in a spring-rate-like manner and increases as the presser elements move further away from the mid-travel datum line as in the case when laying down and compacting over a convex surface. Since the common sleeve induces a sleeve resistance on the presser elements as they move it is difficult to maintain a uniform pressure gradient across the compaction line as the presser member lays and compacts the fiber tows over a varying contour. The common sleeve effectively decreases the compaction force output capability of the presser elements thus causing the compaction line force to be nonlinear with presser element position. In the case of a presser member having compaction rollers moving with respect to a datum centerline, the sleeve resistance is unsymmetrical about the center line of the segmented presser member and increases with each presser element location moving away from the center line in either direction. A second problem is caused by the stretching or bulging of the common sleeve. As the presser elements move to conform to the changing contour of the work surface they induce a shearing-like force into the sleeve. This shear-like force causes the sleeve to bulge out and not maintain a tight fit to the presser element. The sleeve distortion or bulging may affect the incoming fiber tow feed path and tend to produce degraded lap/gap within the tow band, tow wander at end cuts and degraded end cut accuracy that may, in turn, lead to imperfections or defects in the composite plastic article and thus to unreliable and scrap articles, as well as increased costs and lower production.